Imputation correctness milestone (audit Wave 1 + tail): MI-properness warnings,
ordered-factor preservation, keep_all_columns, irmi(mi>1) list return,
imputeRobust/imputeRobustChain repairs, cellwise cellIRWLS/init_weights fixes,
kNN/gowerD mixed-scaling + weightDist guard + NA-convention docs. R CMD check
clean. Stable reference point for the R Journal (vimpute) paper.